Ca promotes photosynthesis
Our results indicated Ca deficiency significantly inhibited P. massoniana photosynthetic process, while sufficient Ca supply markedly facilitated photosynthesis (Figure 1). Consistently, proteomic analysis revealed the down-regulation of a plenty of photosynthesis related proteins under low Ca level and the recovered and even increased expression of these proteins under high Ca level, including both light reaction and dark reaction related proteins (Table 1). PPI network analysis indicated photosynthesis is an important function module of Ca influenced physiological process (Figure 5). A previous review has well characterized the role of Ca in plant photosynthesis.15 Our study further consolidated the important role of Ca in photosynthesis and demonstrated the mechanism of Ca regulated photosynthesis inP. massoniana .
PsbP family protein, one of the key components of photosystem II in higher plants, can enhance oxygen evolution rate at physiological Ca and chloride concentration.32 The high Ca treatment activated the up-regulation of Mog1/PsbP/DUF1795-like photosystem II reaction center PsbP family protein (spot 14), suggesting photosynthetic oxygen evolution may be accelerated by adequate Ca supply. The changed expression abundance of chlorophyll A/B binding protein (spot 15), plastid high chlorophyll fluorescence 136 precursor (spot 19), flavoprotein WrbA-like (spot 23) and oxygen-evolving enhancer protein 1 (spot 25) imply other light reaction processes could also be affected by Ca level.
Ribose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is the most abundant cellular soluble protein in plant, catalyzes the initial step of CO2 fixation and is a rate-limiting enzyme for CO2assimilation.33 The down regulation of a variety of RuBisCO related proteins (spots 13, 17, 20, 22, 24, 26, 27, 28) indicated CO2 fixation and assimilation potentially be hindered under Ca deficiency condition. High Ca treatment dramatically reversed this situation; significant up-regulation of these proteins were observed, suggesting the promoting role of Ca in CO2 assimilation. Chloroplast stroma located carbonic anhydrase (CA), the second richest plant cellular protein apart from RuBisCO, catalyzes the inter-conversion between CO2 and HCO3- and enhance the delivery of CO2 to RuBisCO.34 It is generally up-regulated at moderate stress severity to respond stress though partially compensating for decreased CO2-conducting aquaporin.35Accordingly, we observe the interaction among CA1, Rubisco large subnunit (RBCL) and Rubisco small subnunit (RBSL) in the constructed PPI network (Figure 5). In the presented study, the up regulated CA1 (spot 11) suggests Ca deficiency bring severe CO2 fixation obstruct in P. massoniana seedlings. High Ca treatment lead to the down-regulation of CA1, which is a naturally result of high Ca restored normal photosynthetic carbohydrate fixation.
Glutamate-glyoxylate aminotransferase (GGAT) is a critical enzyme engaged in photorespiration, which is an energy and organic carbohydrate consumption process in plant.36 Ca deficiency possibly increased photorespiration while inhibited photosynthesis as significantly up-regulation of GGAT 2-like protein were observed at low Ca level. High Ca may lower photorespiration by down-regulating GGAT related proteins.
Collectively, Ca deficiency impaired the photosynthesis process inP. massoniana seedlings. Adequate Ca supply improved photosynthesis through promoting CO2 fixation, regulating light reaction and inhibiting photorespiration processes.